I have a question for you that I've always wondered about the answer, but I do not know anyone to which to pose the question. How accessible is a basic, plain

I have a question for you that I've always wondered about the answer, but I do not know anyone to which to pose the question. How accessible is a basic, plain vanilla, semantic HTML document? I.e., a fully server side rendered HTML doc that uses paragraph and heading and so forth tags for what they semantically mean. Are those type pages accessible, or is more work required to make them accessible?